Influence of Copper Thickness on the Defect Formation of CdTe Solar Cells

Thanks to the high efficiencies achieved and to the low cost of industrial production, CdTe photovoltaic panels are still the main and most promising alternative to traditional silicon cells. In the last few years a solar cell record efficiency of 22.1 % and module efficiency of 18.6% have been achieved, but there is still room for improvement. Actually the attainment of a larger hole density by doping would improve the open circuit voltage and consequently the efficiency. Until now copper has been considered the main dopant for CdTe cells: Cu dopes CdTe through the formation of substitutional impurities CuCd, but it also acts as a donor in interstitial positions, resulting in reduction of hole density. Moreover Cu is a fast diffuser in CdTe and it is widely considered the main cause of degradation of these devices. The doping and degradation mechanisms due to copper are still unclear, which is why they remain a hot topic in CdTe devices. In this work we show that copper thickness in the back contact of CdTe thin film solar cells affects their performance and stability. In particular, we have observed that different copper thicknesses lead to different predominant degradation mechanisms. A study on how the amount of copper influences the defects formation in the absorber has been conducted.